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The amplifier is extremely sensitive. I had your problem and the only way I resolved it was to put the amp + sensor in its own metal box. Note the CRO can cause the problem as well. Best to use the LCD display to gauge how its working. Forget reading the elektor document and gives no hints.

JohnneyBoySo the test are as follows, the op amp is putting out 800Hz (square wave, not 100% 50/50 duty cycle, about 60/40) ! on the mains psu (9v)

If I put the unit on a battery, I get 630Hz.

There shouldn't be any repetitive signal, and most certainly not a square wave, on the opamp's output. Did you make a solid connection from the aluminum wrap to the circuits ground?Although I doubt it, could it be some sort of oscillation of the opamp circuitry? Can you measure the opamp's input? And is the power supplied to the preamplifier PCB clean? Perhaps it's the 7805?

I have found the issue, it seems the plated though hole on R3 to ground is not plated through! I have soldered both sides of it, and R4 just in-case and now the unit seems to be working. I wonder how many more holes have the same issue.

The only issue is where should the Level be set, how does one calibrate it?

Excellent that you found it, not so excellent that the hole wasn't plated through, though. It surely is a production error on your board. It's the first time I've heard something went wrong in that department.

Calibrate is a big word. Background radiation can be used, and should yield 0.33 counts per minute.

Hey,I too have a some issues with the radiameter.Should say, I'm pretty much a noob in the world of electronics.FirstAs you can hardly see (shouldn't have used a flash ), the dots around the text stays visible, is this normal? I dont see this with other photos?

Second problem, is that even without the sensor installed, it keeps on counting. Checked every connection, seems fine..The wires to connect the display aren't done very nice - should say that previously these were done properly and had exactly the same problem.

any ideas?

I work in radiaton security, so we are kinda curious how good or bad it would work.

From light it was definitely shielded.Last thing I tried was to use an aluminium mylar to cover the sensor, and connect that mylar to -This gave the same results.So I thought, maybe it is better if the wiring was smaller between board and sensor, but one of the pins of the diode broke during process.To be continued.